The present invention relates to data communication, specifically to a delay-in-frames correcting system in a PCM transmission line for correcting delays of data in frames at a switch unit in the PCM transmission line.
The system configuration shown in FIG. 1 is used in transmitting data between a subscriber terminal and another subscriber terminal in wide-band data transmission using, for example, 64 Kbps.times.N (N equals 1-30 channels) in a PCM transmission line (2.048 MHz in Europe, and 1.544 MHz in the U.S. and Japan).
In FIG. 1, subscriber terminals A and B are connected to a PCM transmission line terminal unit (DT) 1 through a PCM transmission line L. 2 is a digital terminal common (hereinafter abbreviated as DTC). A delay-in-frames arises at a Switch unit 3 having a configuration T-S-T, for example.
The above described switch unit 3 comprises a multiplexer (MPX) 4 for multiplexing a transmission signal, a primary time switch TSW1, a space switch SSW, a secondary time switch TSW2, and a demultiplexer (DMPX) 5. The primary time switch TSW1 and the secondary time switch TSW2 comprise memories MEM1 and MEM2 individually.
The above described PCM transmission line L is set to 2.048 MHz in Europe and 1.544 MHz in the U.S. and Japan, carrying 64 Kbps data per channel (ch). This enables the data transmission of 64 Kbps.times.30 channels at maximum in Europe and 64 Kbps.times.24 channels at maximum in the U.S. and Japan. For example, in transmitting video signals each carrying 384 Kbps data, a signal is transmitted through 6 channels (64 Kbps.times.6=384 Kbps).
FIG. 2A shows the data configuration of 128 channels on the output side of the DTC 2 shown in FIG. 1, that is, the input side of the switch unit 3. In the configuration, control channels TS 0-TS 3 control the PCM transmission line terminal unit 1; TS 4-TS 63 and TS 68-TS 127 are voice channels; and TS 64-TS 67 are call control channels. That is, the data comprises 120 voice channels and 8 control channels.
Eight units of the above described data form one block, and are multiplexed by the multiplexer 4 to finally form 1024 channels shown in FIG. 2B. Then, they are switched through the primary time switch TSW1, the space switch SSW, and the secondary time switch TSW2, demultiplexed by the DMPX 5, and transmitted to the subscriber B through the DTC 2 and the DT 1. The path of this signal is shown in FIG. 1 as a bold line.
The method of transmitting data from a terminal to the input side of the primary time switch in the switch unit 3 is described below in detail. A line is connected to each of the digital terminals 1 shown in FIG. 1, and the digital terminal common 2 multiplexes/demultiplexes the data applied from a plurality of lines. In Europe, 1 line equals 30 channels, and 4 lines are connected to the digital terminal commons 2. In the U.S., 1 line equals 24 channels, and 5 lines are connected to the DTCs 2.
The DTCs 2 are operated as an interface between the digital terminals 1 and the switch unit 3. They multiplex/demultiplex data for the voice channels, transmit an operation mode of the DT1 through the control channel, and notify the switch unit 3 of the alarm state of the PCM transmission line L.
Each memory in the primary time switch TSW1 and the secondary time switch TSW2 has the capacity of 1028 channels and receives data consisting of 128 channels from each of the DTCs 2. Therefore, the MPX 4 and the DMPX 5 shown in FIG. 1 multiplex/demultiplex signals for eight DTCs 2.
In the flow of a signal described above, data may delay in frames at the switch unit 3. Assume that, for example, information comprising three data A, B, and C are applied to the switch unit 3. When the information is switched and outputted, the data in channel C can delay and enter the frame next to its own frame. The delay may be caused by an erroneous allocation of the memory MEM1 in the primary time switch SW1 or the memory MEM2 in the secondary time switch SW2.
Thus, information comprising three data pieces A, B, and C often becomes meaningless if they suffer delays in their frames during the switching operation.
FIG. 3 shows how the delays-in-frames take place. In FIG. 3, .alpha..sub.1 indicates the state of each signal on the input side of the primary time switch TSW1; .alpha..sub.2 indicates the state of each signal between the primary time switch TSW1 and the secondary time switch TSW2; and .alpha..sub.3 indicates the state of each signal on the output side of the secondary time switch TSW2, wherein frames are named frame N, frame N+1, frame N+2, . . . , each comprising 1024 channels from channel 0 to channel 1023.
In FIG. 3, data A in channel 0 in the state .alpha..sub.1, for example, delay a little in phase and enter the second channel in the state .alpha..sub.2. Then, they enter the 514th channel in the state .alpha..sub.3. However, they are still in frame N, the same frame as that in which the data were inputted. In the meantime, data B in the second channel in the state .alpha..sub.1 enters frame N+1 in the state .alpha..sub.3, and data C in the 510th channel in the state .alpha..sub.1 enters frame N+2 in the state .alpha..sub.3. Thus, they delay and enter the frames different from those where they were in when inputted.
As described above, data can be detected in the different frames on their input and output sides of the switch unit 3.
In FIG. 3, the output frame of the secondary time switch TSW2 leads by 513 time slots because the upstream data and downstream data are allocated separately in the switch unit. FIG. 4 shows the difference in the allocation. Assuming that, in FIG. 4, the transmission line from terminal A to terminal B is an upstream line and that from terminal B to terminal A is a downstream line, a channel allocated the state .alpha..sub.2 in the upstream path is allocated .alpha..sub.2 +512 in the downstream path. Therefore, the output side of the upstream path leads by 513 time slots.
When data are transmitted in the PCM transmission line through the wide-band of 64 Kbps.times.Nch (N equals 30 channels in Europe, and 24 channels in the U.S. and Japan), video data are transmitted through 64 Kbps.times.6 channels, wherein the data in 1-6 channels must be transmitted at the same timing. Therefore, as described above, a delay of data in frames caused in the switch unit 3 must be corrected appropriately.
To correct the delays, the prior art technology controls such that the necessary number of channels are retained in order and a significant data string is outputted in the same frame without data delays in frames under software control.
However, in this method, channels cannot be allocated such that they are retained in order with a significant data string outputted in the same frame without delay when a number of calls are required, thereby causing a problem of an unpractically high call loss rate. A "call loss rate" means a rate of calls which cannot be connected to a receiving side due to the congestion at a switch unit, etc., that is, a rate of channel allocations which fails in retaining the order of data.